Body size is of fundamental importance in understanding macroevolutionary patterns, both for extant taxa and for those with a fossil record. In this paper we describe four different kinds of body-size evolution: autapomorphic giantism, autapomorphic nanism, phyletic giantism, and phyletic nanism. The terms giantism and nanism are preferred here rather than the frequently, although incorrectly used equivalents, gigantism and dwarfism, respectively. We assert that without a known phylogeny, it is difficult or impossible to differentiate these four different kinds of body-size evolution. Case examples are presented for two groups: varanid lizards (family Varanidae) and fossil horses (family Equidae).
Previous hypotheses of body-size evolution within the Varanidae suggested that there were several cladogenic events in which some groups and isolated species became large. The most recent phylogeny of Varanidae based on mtDNA suggests otherwise. Mapping the known total body lengths onto the phylogeny indicates that varanids were already getting large early in their evolutionary history, with the crown group, Odatria, becoming secondarily small on mainland Australia. Although hypothesized as a giant island varanid, the komodo dragon (Varanus komodoensis) is discovered to be nested within a clade in which the basalmost taxon (V. salvadorii), also endemic to an island, reaches body lengths similar to those of the komodo dragon. Review of the Varanidae suggests that caution should be taken when characterizing taxa as island giants/dwarfs without first reviewing a phylogeny.
Fossil horses (family Equidae) are frequently cited in the literature, as well as depicted in museums, as prime examples of Cope's rule, i.e., a gradual trend toward body-size increase over time. Several recent parsimony analyses have resolved many of the phylogenetic interrelationships of North American fossil horses and have elucidated their patterns of body-size evolution. In light of these new analyses, there is no evidence for Cope's rule in fossil horses. In fact, the evolution of large body size occurred multiple times in fossil horses and exemplifies autapomorphic giantism. Body-size decrease, oftentimes considered the exception to Cope's rule, is actually widespread within multiple clades of fossil horses and is characterized by both autapomorphic and phyletic nanism.
The result of our analysis suggests that studies of body-size evolution must be intimately tied to a phylogeny before distinct patterns, if any, can be discerned. Cope's rule is not applicable to the two case examples presented herein, calling into question the most frequently cited mode of body size evolution.
